The present invention relates to an image forming apparatus, such as a printer, a copy machine, a facsimile and so on.
The conventional image forming apparatus, such as a printer, a copy machine, a facsimile and so on, uses a so-called electrophotographic process, in which the surface of a photo conductor operating as an image carrier is uniformly charged, an image is exposed on the charged photo conductor surface for producing an electrostatic latent image thereon, a developer is supplied to the photo conductor carrying the electrostatic latent image for developing the electrostatic latent image, and a toner image thus formed on the photo conductor is transferred to a sheet of paper, an OHP sheet or a recording body, such as an intermediate transfer body, to obtain a printed image.
In an image forming apparatus of this kind, for the purpose of stably reproducing images with a high image quality over a long period of time, there is a known mechanism in which a patch is formed on the surface of the image carrier body before initiation of the printing operation, after the printing operation or during the printing operation, and various parameters associated with printing are controlled on the basis of the information obtained from the patch. Here, in the patch employed for such control, there are techniques for performing control using a xe2x80x9ctoner patchxe2x80x9d formed by depositing toner on the image carrier body and a technique for performing control using a xe2x80x9cpotential patchxe2x80x9d formed as a latent image, without depositing any toner.
In the case of the toner patch system, since a toner image has to be formed on the image carrier body, an extra amount of toner is consumed. Furthermore, since the toner patch subsequently has to be removed from the image carrier body, the load on the cleaning device of the apparatus tends to be increased.
In contrast to this, in the case of a potential patch, it is sufficient to form the latent pattern on the image carrier body during the charging step and exposure step to solve the problems set forth above. In this type of system, as disclosed in Japanese Patent Application Laid-Open No. Heisei 9 (1997)-230688, it is typical to provide a potential sensor for detecting the potential patch between the exposure device and the developer device for detecting the potential upstream of the developer device. However, when the a printing speed of the image forming apparatus is increased, a greater amount of developer has to be supplied to the image carrier body. As one approach to satisfy this requirement, a multiple stage developing roller system having a plurality of developing rollers has been employed. However, when such a multiple stage developing roller type developing device is employed, which results in an increase in the size of the apparatus, a difficulty is encountered in providing sufficient space for mounting a potential sensor between the exposure device and the developing device.
In addition, mounting the potential sensor between the exposure device and the developing device also is not always appropriate simply from the view point of increasing the printing speed. Namely, it is possible that the potential patch may pass below the potential sensor before the potential of the exposure portion drops down (decays) to the predetermined potential, due to the optical response characteristics of the image carrier body (photo conductor), thereby making it impossible to accurately detect the potential patch.
Accordingly, an object of the present invention is to provide an image forming apparatus which restricts disturbance of a potential patch, in the case where a potential sensor for detecting the potential patch is provided on the downstream side of a developing device of a multiple developing roller type, thereby to enable stable reproduction of a high quality image for a long period of time.
In order to accomplish the above-mentioned and other objects, according to a first aspect of the present invention, an image forming apparatus comprises: charging means for charging the surface of an image carrier; exposure means for exposing an image on the charged image carrier for forming a latent image; developing means, including a plurality of developing rollers arranged in opposition with a surface of the image carrier and bias applying means for applying a developing bias to the plurality of developing rollers, for supplying a developer on the image carrier and forming a developed image on the image carrier; transfer means for transferring the developed image formed on the image carrier onto a printing medium; a potential sensor provided downstream in the moving direction of the image carrier relative to the developing means for detecting a potential on the image carrier; and control means for setting the developing bias to a value restricting disturbance of a potential portion, representing an object for potential detection by the potential sensor, by the developer when the potential portion passes across the developing means.
Preferably, setting of the developing bias for the plurality of developing rollers may be performed in sequential order from the developing roller arranged on the upstream side in the moving direction of the image carrier.
According to a second aspect of the present invention, an image forming apparatus comprises: an image carrier; charging means for charging the surface of the image carrier; exposure means for exposing an image on the charged image carrier for forming an exposure portion potential region; developing means, including a plurality of developing rollers arranged in opposition with a surface of the image carrier, bias applying means for applying a developing bias to the plurality of developing roller and a two component developer, for supplying a developer on the image carrier and forming a developed image on the image carrier; transfer means for transferring the developed image formed on the image carrier onto a printing medium; a potential sensor provided downstream in the moving direction of the image carrier relative to the developing means for detecting a potential on the image carrier; and control means for setting the developing bias to a value for restricting deposition of toner on the exposure portion potential region when the exposure portion potential region passes across the developing means.
According to a third aspect of the present invention, an image forming apparatus comprises: an image carrier; charging means for charging the surface of the image carrier; exposure means for exposing an image on the charged image carrier for forming an exposure portion potential region; developing means, including a plurality of developing rollers arranged in opposition with a surface of the image carrier, bias applying means for applying a developing bias to the plurality of developing rollers and a two component developer, for supplying a developer on the image carrier and forming a developed image on the image carrier; transfer means for transferring the developed image formed on the image carrier onto a printing medium; a potential sensor provided downstream in the moving direction of the image carrier relative to the developing means for detecting a charge potential and an exposure potential on the image carrier; and control means for applying the developing bias at a value which restricts the splashing of carrier onto the surface of the image carrier, when the charge potential region passes through the developing means, and sets the developing bias to a value for restricting deposition of toner onto the exposure portion potential region when the exposure portion potential region passes across the developing means.
According to a fourth aspect of the present invention, an image forming apparatus comprises: an image carrier; charging means for charging the surface of the image carrier; exposure means for exposing an image on the charged image carrier for forming an exposure portion potential region; developing means, including a plurality of developing rollers arranged in opposition with a surface of the image carrier, bias applying means for applying a developing bias to the plurality of developing rollers and a two component developer, for supplying a developer on the image carrier and forming a developed image on the image carrier; transfer means forming a transfer nip portion by contacting the surface of the image carrier and transferring the developed image formed on the image carrier onto a printing medium in the transfer nip; a potential sensor provided downstream in the moving direction of the image carrier relative to the developing means for detecting a charge potential and an exposure potential on the image carrier; and control means for applying the developing bias at a value which restricts the splashing of carrier onto the surface of the image carrier when the charge potential region passes through the developing means, and sets the developing bias to a value for restricting deposition of toner onto the exposure portion potential region when the exposure portion potential region passes across the developing means.
In the preferred construction, the developing bias may be reduced in sequential order from the developing roller arranged upstream in the moving direction of the image carrier upon reducing the developing bias of a plurality of developing rollers. The developing bias also may be applied in sequential order from the developing roller arranged upstream in the moving direction of the image carrier upon applying a developing bias to a plurality of developing rollers.
The image forming apparatus may further comprise: layer thickness detecting means for detecting the layer thickness of the image carrier; a humidity sensor for detecting the humidity around the image carrier; and dark decay storage means for storing a value representing the potential drop due to dark decay of the image carrier corresponding to detection values of the layer thickness detecting means and the humidity sensor, whereby at least one of the charge voltage level of the charging means and the light output of the exposure means is corrected on the basis of the potential drop derived from the detection values of the layer thickness detecting means and the humidity sensor.
According to a fifth aspect of the present invention, an image forming apparatus comprises: an image carrier; charging means for charging the surface of the image carrier; exposure means for exposing an image on the charged image carrier for forming an exposure portion potential region; developing means, including a developing roller arranged in opposition with a surface of the image carrier, bias applying means for applying a developing bias to the developing roller and a two component developer, for applying the developer held on the developing roller to the surface of the image carrier at a developing nip for supplying developer to the image carrier to form a toner image on the image carrier in the developer nip; transfer means for transferring the toner image formed on the image carrier onto a printing medium in a transfer nip; a potential sensor provided downstream in the moving direction of the image carrier relative to the developing means for detecting a charge potential and an exposure potential on the image carrier; and control means for setting the developing bias to a value for restricting deposition of toner onto the exposure portion potential region when a tip end of the exposure portion potential region reaches a rear end of the developing nip in the moving direction of the image carrier.
The image forming apparatus preferably comprises means for controlling the potential of an image region, on the basis of a detection value of the potential sensor, to maintain the potential constant, detecting the layer thickness of a photo conductor layer forming the image carrier, and controlling the peripheral electric field of the image region.
The image forming apparatus may include: a first potential sensor arranged within a range from the developing means toward the charging means in the moving direction of the image carrier, and a second potential sensor arranged within a range from the charging means toward the developing means in the moving direction of the image carrier, wherein the potential of the charge potential region is controlled so as to be constant on the basis of a detection value of the second potential sensor, and the layer thickness of the photo conductor is detected on the basis of a detection value of the first potential sensor.
The image forming apparatus may employ an auxiliary exposure for controlling the peripheral electric field. For this purpose, an auxiliary exposure light is irradiated at a position of transition from a potential of the charge potential region to the exposure potential region for forming a step in the potential distribution. At least one stepwise potential distribution may be formed between the developing bias voltage and the potential of the charge potential region.
The image forming apparatus may further comprise means for detecting the potential of an image region, where a latent image exists, using a potential sensor, controlling the potential of the image region, other than a solid image region, among the image regions, on the basis of detection values thereof, detecting a layer thickness of the photo conductor, and controlling a peripheral electric field of the image region, including the solid image.